1. Field of the Invention
This invention relates to current multiplier circuits and, more particularly, to a balanced multiplier circuit for use with a subscriber loop interface circuit (SLIC) for balancing a load current supplied by the SLIC.
2. Description of the Prior Art
SLICs suited for fabrication as an integrated circuit that are designed to replace the hybrid transformer circuit generally used in telephone line central office equipment, PABX and Subscriber carrier equipment providing signal separation for two-wire differential to four-wire single-ended conversions and suppression of longitudinal signals at the two-wire input are well-known. For example, U.S. Pat. No. 4,004,109 describes such a SLIC. Moreover, a SLIC providing the functions described above is manufactured by Motorola, Inc. as a standard product identified as the MC3419. Furthermore, the operation of the MC3419 is disclosed in U.S. Pat. No. 4,300,023 which is assigned to Motorola, Inc.
Besides providing AC signal conversion and longitudinal signal suppression, the SLIC must provide a DC current to the two-wire subscriber loop to power the particular load coupled thereto. For instance, if a telephone is connected to the two-wire inputs of the SLIC, the DC line current powers up the telephone. One significant aspect then of the SLIC is to maintain the mid-point voltage of the load centered with respect to the power supply voltage of the SLIC. This may be necessary to prevent distortion of the AC signals appearing across the Tip and Ring terminals (the two-wire inputs) of the SLIC which otherwise may occur if the load voltage was skewed towards either side of the power supply voltage.
Another important feature of the SLIC is to provide suppression of longitudinal or common mode signals that appear at the two-wire input thereof. There is nearly always some longitudinal signals present due to the coupling of the subscriber load equipment via telephone lines to the Tip and Ring inputs of the SLIC. Most SLICs include circuitry for suppressing longitudinal signals. One problem with some SLICs is that these SLICs can operate correctly only if the longitudinal signal currents do not exceed the loop current supplied by the SLIC. Since some subscriber equipment, such as a telephone, may be located some distance from the central office located SLIC, the telephone line resistance reduces the available line current that can be supplied by the SLIC. Unfortunately, longitudinal signal currents generally increase with extended telephone lines. Hence, it is possible, using contemporary SLICs, to have longitudinal signals greater than can be handled by the SLIC which is undesirable.
Additionally, some contemporary SLICs require Darlington transistors externally coupled between the two-wire inputs and the SLIC to provide sufficient DC current multiplication to the subscriber loop. Thus, the subscriber loop and the SLIC operate as a high current gain loop and as such may be subject to oscillations as understood. To prevent oscillations, SLICs generally require a pair of frequency compensation capacitors. The need for such capacitors not only increase component count but is an expense to the user of SLICs which is desirable to eliminate.
Still further, some SLICs, as described above, have an internal circuit for centering the load voltage which is typically referred to as a "C-circuit". This internal circuit also provides a high gain loop which requires an additional external pin for coupling to additional frequency compensation capacitor. It is desirable to eliminate external pin count as well as external required capacitors to reduce expense of the SLIC to the user.
Thus, there exists a need for a SLIC to overcome the aforementioned problems generally associated with contemporary integrated SLICs.